In detail, the present invention relates to the field of transmit antenna diversity. A rather comprehensive introduction to the technical field of diversity is for example given by Juha Korhonen in “Introduction to 3G mobile communications”, chapter 3.4., page 86 to 94, Artech House mobile communications series, 2001. More specifically, the concern of this invention resides in a mode selection procedure by which a suitable transmit diversity mode for each link in the cell defined by the coverage area of the subject transmitter can be selected.
It is to be noted that while reference is made to an antenna array, an array comprising a single antenna only may still be regarded as an antenna array. Likewise, an antenna and/or antenna element of the array produces a beam of electromagnetic radiation in operation, i.e. when being driven, and also terms “antenna” and “beam” are used interchangeable, as the antenna configuration and driving will determine the produced beam. It is to be noted further that when considering diversity, a diversity branch can also be represented by the beam produced by the corresponding (diversity) antenna array.
To recapitulate, there are different transmit diversity modes in FDD WCDMA (FDD=Frequency Division Duplex, WCDMA=Wideband Code Divisional Multiple Access) dedicated downlink channels:    1) an open-loop diversity mode using space-time codes such as for example the concept known as STTD (Space-Time Transmit Diversity), and    2) a closed-loop diversity mode, which can be classified into different closed-loop diversity classes:    a) a first class, subsequently referred to as class 1, in which the receiver (e.g. a user equipment UE) returns information to the diversity transmitter (e.g. a Node_B) concerning the relative phase of the received diversity transmission signals; an example for such a class 1 closed-Loop diversity mode is known as closed-loop loop mode 1; and    b) a second class, subsequently referred to as class 2, in which the receiver returns information to the diversity transmitter concerning the relative phase and the ratio of received powers of the received diversity transmission signals; an example for such a class 2 closed-loop diversity mode is known as closed-loop mode 2 (as for example described in the above cited book by Juha Korhonen or as described by 3GPP TS 25.214: “Physical Layer Procedures (FDD)”).
It is to be noted that closed-loop diversity is only applicable to a downlink channel if there is an associated uplink channel which is required to return the feedback information. An example for such a channel combination is represented by the DPCH (Dedicated Physical CHannel)/DPCCH (Dedicated Physical Control CHannel) channels.
First, the present transmit diversity methods in WCDMA are recalled. The present (standardized) Transmit Diversity Methods in FDD WCDMA dedicated downlink channels are, as mentioned herein before, for example    STTD (Space-Time Transmit Diversity) as an open-loop solution utilizing a simple 2×2 space-time code,    closed-loop mode 1 (CL1) as an example for class 1 closed-loop diversity according to which the relative phase between transmitted signals is adjusted based on the feedback from the receiver (user equipment), and    in closed-loop mode 2 (CL2) as an example for class 2 closed-loop diversity according to which both, relative phase and power between transmitted signals, are adjusted based on the feedback from the mobile.
The Node_B as the transmitter (corresponding in its functionality to a base station BS known from GSM system) can select the mode to be used for each link separately or it can use the same mode for all links in the cell. In this invention, however, the former case is studied, since the latter solution is not recommended because it does not utilize the available capacity potential.
Generally, the mode selection problem resides in finding measures and a method by which a best suitable mode can be selected for each link. A “best suitable mode” can be determined as the mode which reveals the best performance (e.g. lowest bit error rate, lowest S/N (signal to noise) ratio, or the like).
The mode selection problem is linked to the employed antenna solution at the transmitter. The present transmit diversity modes are designed by assuming that the average received powers at the receiver side (UE) from separate transmitter (Node_B) antennas are the same. Thus, if average power of signals received at the UE and originating from an antenna Ant1 is denoted by P1 and average power of signals received at the UH and originating from antenna Ant2 is denoted by P2, so that the ratio thereof, P1/P2=1. When this assumption holds, the performance of the closed-loop schemes depends in a certain manner on the feedback delay and on the spatial correlation between transmit antennas. If the ratio P1/P2 is not equal to unity, then the sensitivity of closed-loop modes to the feedback delay and spatial correlation between transmit antennas will be different than in case P1/P2=1. This can be illustrated by a simple example. If transmitter selection combining is used, then the signal is transmitted trough the antenna which provides better channel. Feedback information from a mobile directs the transmit antenna selection. If P1/P2=1 in the receiver, then both received channels are in good state with equal probability and system performance depends on the feedback delay and transmit antenna correlation. If feedback delay is large when compared to channel coherence time, then system performance is corrupted. Similarly, if transmit antennas correlate heavily, then both antennas are in good and bad states simultaneously and the lack of diversity corrupts the system performance. However, if P1/P2 is all the time very high—or very low—then the same antenna provides better channel almost all the time and feedback delay or antenna correlation are not corrupting the system performance much. The effect of P1/P2 is not the same to all transmit diversity modes and therefore it should be taken into account when transmit diversity mode is selected.
In FIG. 1 there is a sketch concerning to the selection problem, when average received powers from separate BS antennas in MS are equal: areas A, B and C (also referred to as Mapping areas) consist of those spatio-temporal correlation value pairs (spatial correlation SC, temporal correlation TC), for which open-loop mode (e.g. STTD), class 1 closed-loop mode (e.g. CL1) and class 2 closed-loop mode (e.g. CL2), respectively, provide the best performance. Apparently, the open-loop mode (STTD) is the best choice in most cases while the operating area of class 2 closed-mode is marginal: class 2 is suitable on ly when time correlation (TC) is very high. class 1 works well when spatial correlation (SC) is relatively high. It is to be noted that the FIG. 1 is only a sketch.
The assumption that average received powers at the receiver would be equal is well posed, if    (a) Transmit antennas are co-polarized    (b) Transmit antenna separation is not very large (i.e. antennas, are in the same site, and not, for example, in separate buildings).
The latter restriction (b) is not a problem in practice since the diversity antennas are usually in the same mast. However, assumption (a) has some drawbacks.
The well known fact is that two low correlated signals can be obtained using cross-polarized antennas (and/or antenna arrays). This antenna solution is more compact and cheaper than a pair of spatially separated co-polarized antennas/antenna arrays. Moreover, beside these known advantages, the so-called polarization mismatch can be avoided by using cross-polarized antenna arrays antennas. This is explained in the following.
Polarization Mismatch Problem:
In a conventional system, a single vertically polarized antenna is used at the transmitter side. This arrangement is feasible if receiver antennas are all vertically polarized. However, this will not be the case in practice. It may even happen that receiver antenna polarization is flat, a horizontally oriented ellipse, or the like. Then huge polarization mismatch losses can be faced.
Also, different physical environments preserve the transmitted polarization in a different way and thus, the danger of polarization mismatch depends on the environment; in open areas the probability of a serious mismatch is expected to be high.
The known solution to this problem is the use of cross-polarized polarized antenna arrays and/or antennas. In GSM related solutions, the downlink (DL) signal is transmitted from cross-polarized antenna branches with equal power (each individual diversity antenna corresponds to a transmitting branch). The relative phase between the signals transmitted from each branch is randomly rotated in order to avoid the situation where signals would erase each other for a long time (this may happen if antenna brancnes have strong correlation, for example because of a line-of-sight, (LOS) situation). This solution prevents the total mismatch between the transmitter and receiver polarizations.
The same method as used in GSM is, however, not straightforward to be used in WCDMA, since channel estimation at the receiver UE is based on common p-lot channel (CPICH). However, if downlink transmit diversity is used, then different CPICHs are transmitted from separate antenna branches and cross-polarized antennas can be employed. Stated in other words, via each antenna of the transmit diversity antennas a respective CPICH is transmitted.
The present WCDMA standards specify in a detailed manner the allowed transmit diversity modes and hence, if cross-polarized polarized antennas are used in connection with transmit diversity, then the special properties of this antenna solution must be taken into account in the limits given by standards.
Summarizing, it has to be noted that    (a) if co-polarized, spatially separated antennas are used in connection with WCDMA transmit diversity modes, then mode selection can be based on a fixed performance chart such as the one proposed in FIG. 1;    (b) based on compact structure, costs and robustness against polarization mismatch, the use of cross-polarized diversity antennas is a very attractive solution. However, when used, this renders the usage of a fixed performance chart such as the one proposed in FIG. 1 not feasible, as will subsequently be explained.Special Characteristics of Cross-Polarized Antenna Arrays:
Currently, the present transmit diversity modes are studied based on the assumption that the average received powers at the receiver originating from separate transmitter antennas are equal (P1/P2=1). This is, however, not necessarily true for cross-polarized antenna arrays if the XPR (cross-polarization ratio) in the channel is high. This is the case especially in rural environments where orthogonal polarization branches are not mixed well and it has been claimed that XPR is relatively high even in urban outdoor environments (See for example Shapira J. and Miller S. in “A novel polarization smart antenna”VTC, May 2001, or in “Transmission Considerations for polarization-smart antennas”VTC, May 201. See also “Method and System For Improving Communication” by Shapira J. and Miller S., International Patent Application WO 98/39856).
Thus, although there might be low correlation between BS antenna branches, the average received power at the receiver from cross-polarized BS antenna branches can be different (P1≠P2); this renders the usage of a fixed performance chart such as the one proposed in FIG. 1 not feasible, or at least its usage will result in a non-optimum diversity mode selection.